Much recent interest has focused on aircraft operation in the lower stratosphere due to the wide field of regard available from this altitude, and also the relatively benign winds in the lower stratosphere compared to higher and lower altitudes. However, even at this altitude, winds are significant and stationkeeping (maintaining position close to a fixed latitude and longitude) can consume a significant amount of power. This power is primarily supplied from some onboard energy source such as batteries or fuel cells. Some airship designs rely on solar cells and rechargeable batteries, but the fundamental need to expend energy for stationkeeping still remains.
Lighter-than-air aircraft (“airships”) intended to operate in the stratosphere tend to be large since they require a large “lifting gas volume” per pound of gross vehicle weight, compared to airships operating at lower altitudes. In addition to the mass of the hull which comprises the lifting volume, the gross vehicle weight comprises electronics, energy storage, power generation, propulsion, and payload systems, all of which can be heavy. A large airship experiences significant drag when it attempts to maintain a fixed geographic location against a prevailing wind, and this requires a high power output, large and heavy propulsion systems, and significant reserves of energy. All of these factors tend to increase vehicle weight which leads to increased vehicle size. The weights of these supporting systems tend to scale with the square of each linear dimension of the airship (for similar geometry) since they are proportional to drag which is proportional to surface area. The lifting capacity scales as the cube of each linear dimension (for similar geometry) since lifting capacity is proportional to airship volume. Because of this square-cube relationship, we can be confident that a sufficiently large airship, if it can be built, will be able to carry all its necessary subsystems. As it turns out, airships intended for stratospheric operation tend to be very large.
In many cases of interest, the stationkeeping requirement (and its associated power and energy requirements) is a dominant (or the dominant) factor in overall airship design, gross vehicle weight, and cost. As a consequence, it would be beneficial if power requirements for stationkeeping could be reduced.
An interesting observation is that the wind direction in the lower stratosphere is commonly opposite to that of the wind at higher levels. For example, the wind in the lower stratosphere (around 60,000 to 70,000 feet) might be generally easterly for many months, while the wind at 120,000 to 140,000 feet is generally westerly during the same period of time. Meteorological data over the last 50 years indicate that the wind tends to “switch direction” roughly every 14 months, when winds are generally calm, with the switch in direction generally proceeding from high altitude to lower altitude (i.e., so there are short periods of time when this countervailing wind does not occur, but these periods occur when wind speed is generally low). After a switch in direction, the wind tends to remain moderate for many months, then it tends to experience a period of stronger winds for just a few months, followed by a decline to more moderate wind speeds for many months, and eventually another switch in direction. The behavior then repeats.
For a single airship operating at any altitude, the size and weight of the power generation and propulsion systems tend to be driven by the peak wind condition, since that determines drag and required power for stationkeeping. If a way could be found to minimize power generation and propulsion requirements during nominal and peak wind conditions, airships could be made smaller and would generally involve lower cost.
Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements.